put all TLS functionality behind a feature flag

The "Cargo feature" `default-tls`, which is enabled by default, is
added, with all TLS support relying on it. This allows using reqwest but
disabling the `native-tls` dependency, by disabling this feature.

Closes #225

src/connect_async.rs

@@ -1,8 +1,7 @@
 use std::io::{self, Read, Write};
 
 use futures::{Poll, Future, Async};
-use native_tls;
-use native_tls::{HandshakeError, Error, TlsConnector};
+use native_tls::{self, HandshakeError, Error, TlsConnector};
 use tokio_io::{AsyncRead, AsyncWrite};
 
 /// A wrapper around an underlying raw stream which implements the TLS or SSL
@@ -128,4 +127,4 @@ impl<S: Read + Write> Future for MidHandshake<S> {
             }
         }
     }
-}
+}